The use of fuel cells has become of increasing interest in recent years for the application of power generation by means of a stationary installation and for purposes of transportation where the fuel cell is transported with the vehicle. The fuel of these fuel cells is commonly hydrogen that has been produced by reacting a hydrogen-containing fuel, usually a hydrocarbon or a low molecular weight alcohol, over one or more catalysts in a fuel reformer.
There are a number of known processes for generating hydrogen from hydrogen-containing fuels in a fuel reformer. A first known process for conversion of hydrogen-containing fuels to hydrogen is known as “steam reformation”, which is conducted at elevated temperatures. In the case of a hydrocarbon fuel, steam reformation proceeds via the following reaction, which is generally endothermic:CnHm+nH2O?nCO+(m/2+n)H2.
One difficulty with steam reformation is that external heat may be required to drive the reaction forward to produce hydrogen and carbon monoxide. External heat can be supplied to the steam reformation catalyst from a number of sources, and is transmitted to the catalyst bed using heat exchangers. Some of the external heat may be supplied by passing the high temperature reformate produced by the catalytic steam reformation through a regenerative heat exchanger, thereby returning some of the heat of the high temperature gas to the endothermic reforming reaction. Alternatively, the external heat may be generated by combustion of anode off-gases and/or other fuels in a burner. The combustion reaction taking place in the burner can be catalyzed or non-catalyzed. Examples of catalytic and non-catalytic burners are described in U.S. Pat. No. 6,232,005 issued to Pettit.
A second known process for converting hydrogen-containing fuels to hydrogen is known as “partial oxidation”, which proceeds via the following exothermic reaction:CnHm+n/2O2?nCO+m/2H2.
Partial oxidation can be performed at high temperatures (about 1200 to 1500° C.) without a catalyst, or can be performed with a catalyst at much lower temperatures, typically about 500 to 800° C. One disadvantage of partial oxidation is that it produces less hydrogen per molecule of hydrogen-containing fuel than steam reformation, since some of the fuel is consumed by oxidation. Since the oxidation is exothermic, there is no need for the provision of external heat through a heat transfer surface.
A third known process for converting hydrogen-containing fuels to hydrogen is “autothermal reformation”, in which fuel, water and oxygen, usually in the form of air, are reacted in the presence of a catalyst to generate a hydrogen-rich fuel gas. Autothermal reformation can be viewed as a combination of two reactions, an exothermic partial oxidation and an endothermic steam reformation, with the net heat of reaction being determined by the ratios of oxygen to fuel and water to fuel. Generally, these ratios are established so that the net heat of reaction is slightly exothermic, thereby eliminating the need for application of external heat, resulting in a relatively simple system design which makes autothermal reforming attractive for practical applications.
As can be seen from the chemical reactions depicted above, considerable amounts of carbon monoxide are produced during conversion of the hydrogen-containing fuel. To avoid poisoning of the fuel cell, the level of carbon monoxide in the reformate must be reduced to a low level. This is particularly true for proton exchange membrane (PEM) fuel cells, which have a low tolerance for carbon monoxide. Thus, the reformate is typically subjected to at least one “carbon monoxide cleanup” reaction, which preferably comprises one or more water/gas shift reactions and/or a preferential oxidation reaction, in which carbon monoxide present in the reformate is consumed in a catalytic reaction with oxygen or water (steam).
Regardless of the specific conversion process utilized, significant thermal stresses are exerted on fuel conversion reactors, which can have a detrimental effect on durability. Designers of such reactors have therefore sought to reduce thermal stresses in the mechanical design of these units.
There are two conventional design approaches to overcome the problem of thermal stress in a fuel conversion reactor. The first is to reduce the stress levels by permitting thermal expansion of components of the reactor, and the second is to increase the strength of the reactor structure or the materials used in the structure so that the maximum operating stress will not exceed the maximum design strength.
One well known type of heat exchanger that is used in a wide variety of applications including boilers and other high temperature heat exchangers is known as the “tube bundle” structure, also called a “shell and tube” heat exchanger. Reference can be made to sections 3.1.2. and 4.2.3 of the Heat Exchanger Design Handbook, 1998, by G. F. Hewitt for a discussion of this type of heat exchanger. There are a variety of such heat exchangers including a fixed tube sheet or fixed head type. In this type there is an exterior metal shell which can, for example, be cylindrical and mounted within this shell are two spaced apart tube sheets on which a number of tubes are mounted. There are head covers or complete heads or channel covers at each end, which serve as fluid manifolds. With such a heat exchanger, the thermal expansion coefficients of the shell and the tubes during operation can cause a differential movement between them. Excessive movement of this type can cause the tubes to loosen in the tube sheets. One known way for overcoming the problem of differential movements is to provide a shell expansion bellows.
U.S. Pat. No. 5,382,271 issued Jan. 17, 1995 to Industrial Technology Research Institute, describes a compact tube and shell structure for hydrogen generation where a catalyst is used in the water-shift reaction in order to reduce the level of carbon monoxide in the outflowing gases. Two tube sheets are mounted near opposite ends of a cylindrical shell and first and second sets of partition plates are mounted between the tube sheets. A plurality of tubes extend between the tube sheets and through the partition plates. There is a porous metal layer arranged immediately below the upper tube sheet and then catalyst material is arranged below this layer. There is an exhaust gas chamber and an exhaust outlet provided below the bottom tube sheet. Combustible gas flows into the shell body by means of an inlet in the upper end. A feed inlet is located in one side of the shell body just below the upper tube sheet. For certain types of hydrocarbons, a catalyst used for the steam reforming step is placed in the middle section while another catalyst used in the last section just above the bottom tube sheet is for the water-gas shift reaction.
With this known device, combustible gas enters the upper chamber formed in the shell above the upper tube sheet and, after combustion, the exhaust gas at a very high temperature passes through the tubes in order to enter an exhaust gas chamber at the bottom. The heat of the exhaust gas is transferred to the porous metal layer and the catalyst(s) while the exhaust gas passes through the tubes. This heat exchange also decreases the temperature of the exhaust gas. With this known hydrogen generator structure, there can be a thermal expansion problem if the tubes expand at a different rate than the shell as the tubes are apparently rigidly mounted in the tube sheets which in turn are rigidly mounted in the shell.